1. Field of the invention
The invention concerns an adjustable capacity storage tank for liquid product enabling the quantity of liquid entering the tank when it is filled to be determined and the flowrate of the liquid expelled from the tank by displacement of a piston to be monitored.
The invention is more particularly concerned with a storage tank of this kind usable in an installation for spraying a liquid coating product and especially in an installation of this kind in which the coating product is a relatively good electrical conductor, is supplied from a grounded source and is to be applied by electrostatic means.
2. Description of the prior art
U.S. Pat. No. 4,785,760 describes an electrostatic coating product sprayer installation in which an intermediate storage tank insulated from ground is mounted at the end of an articulated arm. In use, the coating product storage tank is filled with just enough product to coat one or more objects to receive the same coating product (in other words, the same color). In one embodiment the storage tank encloses a mobile piston delimiting a first chamber adapted to receive the coating product and a second chamber adapted to receive an actuator fluid, to be more precise compressed air. The coating product is supplied under pressure, enabling the storage tank to be filled by pushing back the piston. During use the storage tank is pressurized by compressed air admitted to the second chamber and the coating product therefore flows progressively from the storage tank to an electrostatic sprayer. In a system of this kind it is necessary to monitor the displacement of the piston both when filling the storage tank, to monitor the quantity of product admitted to the tank, and during application of the coating product because it may be necessary to modify the flowrate at any time depending on the shape of the object to be coated. To achieve this the response time of the means monitoring the displacement of the piston must be as short as possible. In a high throughput installation such as an installation for coating automobile bodies, for example, the response time of the systems for evaluating the contents of the storage tank and for controlling the flowrate of the coating product to the sprayer must be in the order of 70 milliseconds, assuming that the sprayer must be capable of movement at 1.5 meters per second and that the flowrate must be modified during a displacement of not more than 0.1 meter. It is difficult to achieve this with entirely pneumatic control. The problem becomes more complicated if the combination of the storage tank and the associated piston actuator means is required to be compact and light in weight so that it can be mounted at the end of an articulated robot arm.
The invention makes it possible to solve all these problems.